The present invention relates to a method and a system for controlling a plate width in a hot rolling mill, particularly in a rolling mill for use with continuous casting.
In a conventional method for forming pig iron into intermediate products, such as slabs, blooms, or the like, molten steel taken out of a converter is poured into a mold and is subjected to natural cooling and the cast steel is heated in an annealing furnace and subsequently rolled by a blooming mill. Recently, the continuous casting which is advantageous over the above-mentioned conventional technique in energy saving, yield, etc., has been put into commercial application. In the continuous casting, molten steel travels through a water-cooled mold and a cooling zone. The molten steel is thus force-cooled and continuously solidifies into plates. In comparison with the first-mentioned method, the continuous casting has the drawback in that the width of the slab cannot be readily changed for the following reason. Where a material is rolled into a plate of a much smaller width and a plate of a dimension suitable according to the rolling schedule is to be obtained, replacement of a mold is required. But the replacement necessitates halting the continuous casting machine. This will considerably deteriorate productivity.
The drawback can be removed by having a roughing mill or blooming mill directly connected to a continuous casting machine and having the vertical roll gap adjustable over a certain range to permit plate width change. With this arrangement, it is possible to make the best use of the advantages of the continuous casting and to reduce the time of halting of the mill.
Various plate width controlling methods and systems based on the above-mentioned concepts have been proposed. One of the proposed methods is a method in which the vertical roll gap distance is controlled taking account of the forecasted width extension.
Where a plate is subjected to width reduction rolling by a vertical roll stand of a roughing mill and is reduced to a desired thickness by a horizontal roll stand of a roughing mill, the cross section of a plate after rolling by the vertical roll stand has projecting or thickened portions at width-wise ends. The cross section is called a dog bone section. The dimension of the elevated or thickened portions varies in the lengthwise direction. When the plate with the dog bone section is reduced by the horizontal roll stand of the roughing mill, the width of the plate is recovered or extended. The amount of the width extension varies in the lengthwise direction. In none of the conventional controlling methods and devices, exact relationship between the dimension of the thickened portions and the amount of the width extension due to flattening of the thickened portions is taken into consideration, and accuracy in the width of the resultant plate is not sufficient.
There is a rolling method of reducing a plate widthwise by a pair of grooved vertical rolls. In this method the fact that the vertical reduction produces thickened portions and the horizontal reduction generates width extension is not taken into consideration, so that a sufficient width accuracy is not attained.